1. Field of the Invention
The present invention relates to an optical information recording/reproduction apparatus such as an optical disk apparatus. In particular, the present invention relates to a solid immersion lens (hereinafter, abbreviated as “SIL”) for near-field recording having a bottom surface in a shape with which the SIL hardly collides with a surface of an optical disk when the disk is warped or inclined (tilted).
2. Related Background Art
In order to attain a higher recording density of an optical disk, it is required to reduce a diameter of a light spot on a recording surface of the optical disk by shortening a wavelength of light used for recording/reproduction and by increasing a numerical aperture (NA) of an objective lens.
Conventionally, attempts have been made in which the front lens of an objective lens is placed in proximity to a recording surface of an optical disk such that a distance between the front lens and the recording surface is reduced to a fraction (½, for instance) or less of a recording wavelength to construct a so-called SIL, thereby obtaining an NA of one or more even in the air.
For instance, a detailed description thereof is given in “High-density Near-field Readout over 50 GB Capacity Using Solid Immersion Lens with High Refractive Index”.
A conventional technique will be described with reference to FIG. 6, FIGS. 7A and 7B, and FIGS. 8A and 8B.
A construction of an optical pickup for near-field recording according to a conventional example will be described with reference to FIG. 6.
A light beam emitted from a semiconductor laser 3 is converted into a parallel light beam by a collimator lens 4 and enters a beam expander 5.
The beam expander 5 is a lens for correcting a spherical aberration occurred at a rear lens or an SIL 13 of an objective lens to be described later and is constructed such that a distance between two lenses of the beam expander is controllable in accordance with the spherical aberration.
A grating 6 is a member for generating a sub-beam for tracking.
The light beam that has passed through a beam splitter (BS) 7 and a polarizing beam splitter (PBS) 8 passes through a ¼ wavelength plate 9 to be converted from linearly polarized light into circularly polarized light.
A photodetector for receiving a part of the light beam reflected by the beam splitter 7 and controlling an emission power of the semiconductor laser 3 may also be provided.
The light beam, whose course is bent by 90° by an upwardly reflecting mirror 10, enters a rear lens 11 of the objective lens.
The objective lens is composed of the rear lens 11 and the SIL (front lens) 13 and mounted on a two-axis actuator 12 that integrally drives the rear lens 11 and the SIL (front lens) 13 in a focus direction and a tracking direction.
There are two types for the SIL 13, one of which is shown in FIG. 7A and the other of which is shown in FIG. 7B.
In FIG. 7A, a light beam condensed by the rear lens 11 of the objective lens is focused on a bottom surface of an SIL 13-a that is a hemispherical lens.
The light beam is vertically incident on a spherical surface of the hemispherical lens and takes the same optical path to be focused on the bottom surface as in the case where the hemispherical lens is not provided. Accordingly, a wavelength shortened by a reflective index of the hemispherical lens is obtained, which produces an effect of reducing a diameter of a light spot.
That is, when the refractive index of the hemispherical lens is referred to as “N” and the numerical aperture of the rear lens 11 is referred to as “NA”, a light spot having a size of “N×NA” is obtained on a recording surface of an optical disk 14.
For instance, when the rear lens 11 having an NA of 0.75 is used in combination with an SIL that is a hemispherical lens having N of 2, an effective NA (hereinafter referred to as “NAeff”) reaches 1.5.
An allowable thickness error of the hemispherical lens 13-a is around 10 μm, which facilitates mass production. Also, a wide viewing angle can be secured, so it is also easy to provide a sub-beam for tracking.
On the other hand, in FIG. 7B, a light beam condensed by the rear lens 11 is focused on a bottom surface of an SIL 13-b that is a super-hemispherical lens.
The bottom surface of the SIL 13-b is a surface that is spaced apart from a center of the super-hemispherical SIL 13-b by R/N.
When an angle formed by an optical axis and the light beam at the bottom surface is referred to as “θt”, a relation expressed by Expression (1) given below holds true between the angle θt and an angle θi formed by the light beam incident on the SIL and the optical axis.sin θt=N×sin θi  Expression (1)
Here, sin θi is the NA of the rear lens 11, so in view of a fact that the light beam is condensed in the SIL having a refractive index of N, a light spot having a size of N2×NA is obtained on the recording surface of the optical disk 14.
A condition that the light beam can be incident on the SIL 13-b limits the NA of the rear lens 11 to 1/N or less according to Expression (1).
When the SIL 13-b is made of a glass material having N of 2, it is possible to obtain a light spot corresponding to an NAeff of 1.8 even when a rear lens having NA of relatively low, for instance, 0.45 can be used as the rear lens 11.
In this case, however, there is a problem in that an allowable thickness error of the hemispherical lens 13-b is limited to around 1 μm.
In either case of these SILs, a distance between the SIL bottom surface and the optical disk 14 is a fraction or less of 405 nm which is the wavelength of a light source. This is because only the case where the distance between the SIL bottom surface and the optical disk is as small as 100 nm or less allows the light beam to act on the recording surface as evanescent light from the SIL bottom surface to attain recording/reproduction with an NAeff light spot diameter.
In order to maintain this distance, gap servo to be described later is used.
The light beam is reflected by the optical disk 14 to become reversed circularly polarized light, and enters the SIL 13 and the rear lens 11 to be converted into a parallel light beam again.
The light beam then passes through the ¼ wavelength plate 9 to be converted into linearly polarized light in a direction orthogonal to the direction in which the light beam originally traveled, and is reflected by the PBS 8 to be condensed on a photodetector 1 (16) through a condensing lens 1 (15) such that the information on the information optical disk 14 is reproduced.
Meanwhile, among light beams reflected by the bottom surface of the SIL 13, a light beam corresponding to an NAeff of less than 1 which has not been totally reflected is reflected to be circularly polarized light reversed from that at the time of incidence, like in the case of the reflection light from the optical disk 14 described above.
On the other hand, in a case of a light beam corresponding to an NAeff of equal to or more than 1 which is totally reflected, a phase difference δ expressed by an expression given below is generated between a P-polarized light component and an S-polarized light component. Accordingly, the light beam is displaced from circularly polarized light to become elliptically polarized light.tan(δ/2)=cos θi×√(N2×sin2 θi−1)/(N×sin2 θi)  Expression (2)
Therefore, after passing through the ¼ wavelength plate 9, the light beam contains a polarized light component in a direction that is the same as the direction in which the light beam originally traveled.
This polarized light component passes through the PBS 8 to be reflected by the BS 7, and is condensed on a photodetector 2 (18) through a condensing lens 2 (17).
This light beam monotonically decreases as a distance between the SIL bottom surface and the optical disk is reduced, so it is possible to use the light beam as an error signal.
When a target threshold value is determined in advance, it becomes possible to maintain the distance between the SIL bottom surface and the optical disk at a desired distance of 100 nm or less by performing the gap servo.
The gap servo is described in detail in the paper cited above.
Also, this light beam is not subjected to modulation by recording information on the optical disk 14, so it becomes possible to obtain a stable gap error signal regardless of the presence or absence of the recording information.
However, the optical disk 14 is easy to tilt mainly in a disk radial direction due to a change in temperature, humidity, or the like.
The reduced distance between the SIL bottom surface and the optical disk 14 increases a danger that the SIL bottom surface and the optical disk 14 come into contact with each other when a relative inclination exists therebetween.
When the SIL bottom surface and the optical disk 14 contact each other, the SIL bottom surface or the optical disk surface is scratched, which makes it difficult to attain precise information recording/reproduction after that.
FIG. 8A is a bird's-eye view of an SIL according to a conventional example and FIG. 8B is an enlarged view in the vicinity of a bottom surface of the SIL.
According to the paper cited above, a method is devised in order to alleviate the problem described above. More specifically, a side surface of the SIL is cut into a conical inclined surface forming an angle of 70° with an optical axis, and an effective portion of the bottom surface is condensed to 40 μm in diameter.
Despite this method, when the distance between the SIL bottom surface and the optical disk 14 is reduced to around 100 nm, the risk is increased that the SIL bottom surface and the optical disk may contact each other if a relative inclination of 0.28° or more exists.
When the distance between the SIL bottom surface and the optical disk 14 is reduced to around 50 nm, an allowable relative inclination is only 0.14°.
An ordinary plastic-made optical disk substrate tilts by around 0.5° due to a change in temperature, humidity, or the like, which significantly hinders practical use of a recording/reproduction method based on this system.